Threshold value operation circuit, and, and gate circuit, self hold circuit, start signal generation circuit using threshold value operation circuit

ABSTRACT

The invention relates to a threshold value operation circuit which can perform an analogue type threshold value operation, and which can generate an AC signal with a stabilized frequency and duty ratio which is not influenced by input signal level or characteristic changes of the circuit elements and the like. A switching device (SWa) is switched by a switching signal (Sg) from an external AC source, to modulate an input signal (Vi) which is the subject of a threshold value operation. A modulation signal (V 1 ) thereof is level detected by a level detection circuit ( 10 ) having a previously set threshold value, and if within a range of a predetermined level, an AC output (Vo) is generated from the level detection circuit ( 10 ).

TECHNICAL FIELD

The present invention relates to a threshold value operation circuitwhich performs a threshold value operation on a level of an inputsignal, and generates an AC (alternating current) signal when at a setlevel (corresponding to logic value 1), and does not generate an ACsignal when not at the set level (corresponding to logic value 0). Inparticular the invention relates to a threshold value operation circuitwhich can perform an analogue type threshold value operation, and whichcan generate an AC signal of stabilized frequency and duty ratio when aninput signal is a set level, and to an AND gate circuit, a self holdcircuit, and a start signal generating circuit to which this thresholdhold value operation circuit is applied.

BACKGROUND ART

With signal processing in safety equipment and the like, where fail-safecharacteristics are required, the presence of an AC signal is processedas a logic value 1 and the absence of an AC signal is processed as alogic value 0. Finally a threshold value operation is carried out on theAC signal and a judgement is made as to whether or not this is within apredetermined level, and a signal indicating safety or danger is output.

As one example of a conventional threshold value operation circuitapplicable to such fail-safe signal processing, there is for example afail-safe window comparator/AND gate. The circuit construction andoperation of this is shown in for example Trans. IEE of Japan, Vol.109-C, No. 9, September 1989 under the heading “A Structural Method foran Interlock System using a Fail-safe Logic Element having WindowCharacteristics”. Moreover, this is also shown in the specifications ofU.S. Pat. No. 4,661,880, U.S. Pat. No. 5,027,114, and in the disclosureof Examined Japanese Patent Publication No. 1-23006.

Representative circuit examples thereof are shown in FIG. 31(A) and FIG.31(B).

FIG. 31(A) is a window comparator having a level detection function.When a level LI of an input I is in a range(r1+r2+r3)E/r3<L1<(r4+r5)E/r5 (where E is the circuit power sourcevoltage, and r1˜r5 are the resistance values of resistors R1˜R5), thecircuit self oscillates with respective transistors Q1˜Q3 repeating anon/off operation in a process of Q2: OFF→Q3: OFF→Q1: ON→Q2: ON→Q3:ON→Q1: OFF→Q2: OFF→ . . .

Furthermore, FIG. 31(B) is a circuit having a level detection functionand an AND function, with two window comparators as shown in FIG. 31(A)connected in cascade. When the levels LI1, LI2 of the inputs 11, 12 arerespectively in the range of (r1+r2+r3)E/r3<LI1<(r4+r5)E/r5, and(r1′+r2′+r3′)E/r3′<LI2<(r4′+r5′)E/r5′ (where E is the circuit powersource voltage, and r1′˜r5′ are the resistance values of resistorsR1′˜R5′), the circuit self oscillates , and if even one of L1 and LI2goes outside of the abovementioned range, oscillation stops.

Furthermore, as another threshold value operation circuit, there is forexample the one disclosed in Japanese Unexamined Patent Publication No.8-162940, previously proposed by the present applicants. This is shownin FIG. 32.

With the circuit of FIG. 32, if an output X2 from an envelope detectioncircuit A based for example on an AC signal V_(IN) exceeds a powersource voltage Vcc, then due to a switching (on/off) of a photocouplerPC1 which receives an AC signal from a separately provided AC source, aswitching current intermittently flows in a photodiode of a photocouplerPC2 via a current decreasing resistor R. Hence a phototransistor of thephotocoupler PC2 switches (on/off), and an AC signal is generated(output is logic value 1). When an AC signal V_(IN) is not input to theenvelope detection circuit A, the output X2 from the envelope detectioncircuit A becomes approximately the power source voltage Vcc. Hence evenif the photocoupler PC1 switches, a switching current does not flow tothe photocoupler PC2, and an AC signal is not generated (output is logicvalue 0). The envelope detection circuit B voltage doubler rectifies theAC signal due to the switching of the phototransistor of thephotocoupler PC2, and outputs a DC (direct current) signal Y which islarger than the power source voltage Vcc.

However, the threshold value operation circuit which uses the windowcomparator of FIG. 31 is a self-oscillation circuit, and the oscillationfrequency and duty ratio are approximately determined by the on and offdelay time of the respective transistors. Therefore due to changes inthe characteristics of the transistors resulting from variations intransistor characteristics, temperature and humidity and the like, ordue to the input signal level, the oscillation frequency and the dutyratio of the AC output change, so that there is the problem in that astabilized AC output cannot be obtained. Moreover, this threshold valueoperation circuit uses processing outside of the power source limits(when an input signal of a predetermined level greater than the circuitpower source voltage is input, an AC signal is generated), and mustsatisfy relationships where the input signal level and the thresholdvalue of the window comparator both change in proportion to the circuitpower source voltage. Therefore, there is the restriction that the inputsignal oscillation circuit and the window comparator must use the samepower source. Hence the degree of freedom for circuit layout is low.

On the other hand, with the threshold value operation circuit of FIG.32, since the construction is such that the AC signal from an externalAC source is used to give oscillation, the oscillation frequency and theduty ratio are not susceptible to influence from the characteristics ofthe circuit elements or the input level, and hence a stabilized ACsignal can be obtained. However, this circuit confines itself to judgingas to whether or not the input signal is greater than the circuit powersource voltage Vcc, in other words to digital judging for the presenceor absence of the input signal, and does not have an analogue thresholdvalue detection function for the input signal.

The present invention takes into the consideration the above situation,with the object of providing a threshold value operation circuit, whichcan perform an analogue threshold value operation, and which cangenerate an output AC signal of stabilized frequency and duty ratio, andfor which the circuit layout degree of freedom is high. Moreover, it isan object to provide an AND gate circuit, a self hold circuit and astart signal generation circuit to which this threshold hold valueoperation circuit is applied.

DISCLOSURE OF THE INVENTION

Accordingly, with the present invention, there is provided a thresholdvalue operation circuit which comprises; a modulation device formodulating an input signal by means of a switching signal from an AC(alternating current) source, and a level detection device for leveldetecting the input signal by comparing this with a previously setthreshold value, and is constructed such that an input signal subject tothreshold value operation is input to one device of the modulationdevice and level detection device, and an output from the one device isinput to the other device, and which generates an AC signal when basedon a level detection result of the level detection device, a level ofthe input signal subject to threshold value operation is a set level.

More specifically, the construction may be such that the input signalsubject to threshold value operation is input to the modulation device,and a modulation signal modulated by the modulation device is input tothe level detection device, and the level detection device generates anAC signal when the modulation signal is a set level. Furthermore, theconstruction may be such that the input signal subject to thresholdvalue operation is input to the level detection device and leveldetected, and a signal which has been level detected by the leveldetection device is modulated by the modulation device. Moreover, theconstruction may be such that there is provided an AC signal generatingdevice for generating an AC signal which is switched by input of amodulation signal modulated by the modulation device.

With such a construction, an analogue type threshold value operationbecomes possible with respect to the input signal subject to thresholdvalue operation. Furthermore, an AC output which is synchronized with aswitching signal from outside can be obtained, so that an AC output ofstabilized frequency and duty ratio which is not influenced bycharacteristic changes and the like of the circuit elements can beobtained.

With the modulation device the construction may be such that a switchdevice is switched by the switching signal from the AC source, tomodulate an input signal. In this case, if the switch device is aphotocoupler which is switched by input of the switching signal from theAC source, then the AC source side and the input side of the leveldetection device can be isolated. Moreover, if the switch device is atransistor, then switching at high speed becomes possible.

In the case where a transistor is used in the switch device, theswitching signal may have an amplitude of a range between the inputsignal and the voltage of the constant voltage supply line, and a signalwhich does not cross over the threshold value of the level detectiondevice may be used for the switching signal input to the transistor.With such a construction, then even if with a fault in the transistor,the switching signal is input directly to the level detection device,the problem where an AC signal is generated from the level detectiondevice by the switching signal regardless of the input signal does notarise.

The construction may be such that the modulation device incorporates abuffer circuit which takes the input of the AC source switching signal,and supplies this to an input line to which the input signal is applied.

In the case where the level detection device comprises a comparator, theconstruction may be such that the AC output from the comparator istransmitted to an output circuit side via a photocoupler. With such aconstruction, the respective power supplies for the output circuit sideand the threshold value operation circuit side can be made independent,so that the degree of freedom for the layout of the threshold valueoperation circuit is increased.

Furthermore, the level detection device constructed to incorporate aZenner diode and a photocoupler for generating an AC signal which isswitched by an output from the Zenner diode. In the case of thisconstruction, the degree of freedom for the layout of the thresholdvalue operation circuit is increased.

The threshold value operation circuit of the present inventioncomprises: a first Zenner diode for level detecting an input signalsubject to threshold value operation; a switch device which is switchedby a switching signal from an AC source, for modulating an output signalfrom the first Zenner diode; a first photocoupler which is switched byinput of a modulation signal modulated by the switch device, formodulating the input signal subject to threshold value operation; asecond Zenner diode for level detecting a modulation signal of the inputsignal subject to threshold value operation which has been modulated bythe switching operation of the first photocoupler; and a secondphotocoupler which is switched by an output signal from the secondZenner diode, for generating an AC signal, and when the level of theinput signal subject to threshold value operation is a set level, an ACsignal is generated from the second photocoupler.

With such a construction, since the upper limit threshold value and thelower limit threshold value can be set independently, the generation ofan AC output when the Zenner diode is short-circuited can be prevented.

An AND gate circuit which uses a threshold value operation circuit ofthe present invention comprises a plurality of threshold value operationcircuits connected in cascade each of which comprises; a modulationdevice for modulating an input signal by means of a switching signalfrom an AC (alternating current) source, and a level detection devicefor level detecting the input signal by comparing this with a previouslyset threshold value, and is constructed such that an input signalsubject to threshold value operation is input to one device of themodulation device and level detection device, and an output from the onedevice is input to the other device, and which generates an AC signalwhen based on a level detection result of the level detection device, alevel of the input signal subject to threshold value operation is a setlevel, wherein an AC output from a former stage threshold valueoperation circuit is input as a switching signal to a modulation deviceof a latter stage threshold value operation circuit, instead of an ACsource.

Another AND gate circuit of present invention comprises: a thresholdvalue circuit, and outputs the added values for all of the AC inputsignals fro operation circuit which comprises; a modulation device formodulating an input signal by means of a switching signal from an AC(alternating current) source, and a level detection device for leveldetecting the input signal by comparing this with a previously setthreshold value, and is constructed such that an input signal subject tothreshold value operation is input to one device of the modulationdevice and level detection device, and an output from the one device isinput to the other device, and which generates an AC signal when basedon a level detection result of the level detection device, a level ofthe input signal subject to threshold value operation is a set level;and an adder circuit comprising a plurality of rectifying circuits forrespectively rectifying AC input signals, which sequentially adds arectified output from a first stage rectifying circuit to a rectifiedoutput from a next stage rectifying circuit, and outputs the addedvalues for all of the AC input signals from a final stage rectifyingcircuit, wherein the addition output from the adder circuit is suppliedas the input signal subject to threshold value operation of thethreshold value operation circuit.

A self hold circuit of the present invention which uses a thresholdvalue operation circuit comprises two threshold value operation circuitseach of which comprises; a modulation device for modulating an inputsignal by means of a switching signal from an AC (alternating current)source, and a level detection device for level detecting the inputsignal by comparing this with a previously set threshold value, and isconstructed such that an input signal subject to threshold valueoperation is input to one device of the modulation device and leveldetection device, and an output from the one device is input to theother device, and which generates an AC signal when based on a leveldetection result of the level detection device, a level of the inputsignal subject to threshold value operation is a set level, wherein onethreshold value operation circuit is made a trigger circuit and theother threshold value operation circuit is made a hold circuit, and anAC signal which is generated based on a logical sum operation on anoutput from the trigger circuit and an output from the hold circuit isinput to a modulation device of the hold circuit as a switching signalinstead of an AC source.

Moreover, another self hold circuit of the present invention comprises:a threshold value operation circuit which comprises; a modulation devicefor modulating an input signal by means of a switching signal from an AC(alternating current) source, and a level detection device for leveldetecting the input signal by comparing this with a previously setthreshold value, and is constructed such that an input signal subject tothreshold value operation is input to one device of the modulationdevice and level detection device, and an output from the one device isinput to the other device, and which generates an AC signal when basedon a level detection result of the level detection device, a level ofthe input signal subject to threshold value operation is a set level;and an adder circuit comprising two rectifying circuits for respectivelyrectifying AC input signals, which adds a rectified output from a formerstage rectifying circuit to a rectified output from a latter stagerectifying circuit, and outputs the added values of the two AC inputsignals from the latter stage rectifying circuit, wherein the additionoutput from the adder circuit is supplied to an input terminal of thethreshold value operation circuit as the input signal subject tothreshold value operation, and an input terminal of the latter stagerectifying circuit of the adder circuit is made a hold input terminal,and an input terminal of the former stage rectifying circuit is made atrigger input terminal, and the AC output signal from the thresholdvalue operation circuit is rectified and then fad back to an outputterminal of the former stage rectifying circuit.

A start signal generating circuit of the present invention which uses athreshold value operation circuit comprises: a start switch comprising afirst contact point and a second contact point having a complimentaryrelation to each other for on/off switching; a capacitor which ischarged via the first contact point which comes on when the start switchis pressed, and which is discharged via the second contact point whichcomes on when the start switch returns, for generating an output signal;and a threshold value operation circuit which comprises; a modulationdevice for modulating an input signal by means of a switching signalfrom an AC (alternating current) source, and a level detection devicefor level detecting the input signal by comparing this with a previouslyset threshold value, and is constructed such that an input signalsubject to threshold value operation is input to one device of themodulation device and level detection device, and an output from the onedevice is input to the other device, and which generates an AC signalwhen based on a level detection result of the level detection device, alevel of the input signal subject to threshold value operation is a setlevel; wherein the output signal from the capacitor is made the inputsignal subject to threshold value operation of the threshold valueoperation circuit, and the output from the threshold value operationcircuit is made a start signal for a machine or the like.

With such a construction, only when the two contact points of the startswitch are operating normally, and the start signal level is a setlevel, is an AC signal generated from the threshold value operationcircuit, so that a start signal for a machine or a like can begenerated.

Another start signal generating circuit of the present inventioncomprises a first and a second threshold value operation circuits eachof which comprises; a modulation device for modulating an input signalby means of a switching signal from an AC (alternating current) source,and a level detection device for level detecting the input signal bycomparing this with a previously set threshold value, and is constructedsuch that an input signal subject to threshold value operation is inputto one device of the modulation device and level detection device, andan output from the one device is input to the other device, and whichgenerates an AC signal when based on a level detection result of thelevel detection device, a level of the input signal subject to thresholdvalue operation is a set level, wherein an AC output from the firstthreshold value operation circuit is input to the modulation device ofthe second threshold value operation circuit as a switching signalinstead of the AC source, and wherein a power source voltage level issubject to a threshold value operation by the first threshold valueoperation circuit, and an output signal which is generated based on anON operation of an operation button is made the input signal subject tothreshold value operation of the second threshold value operationcircuit, and the output from the second threshold value operationcircuit is made a start signal for a machine or the like.

Another start signal generating circuit of the present inventioncomprises a self hold circuit provided with two threshold valueoperation circuits each of which comprises; a modulation device formodulating an input signal by means of a switching signal from an AC(alternating current) source, and a level detection device for leveldetecting the input signal by comparing this with a previously setthreshold value, and is constructed such that an input signal subject tothreshold value operation is input to one device of the modulationdevice and level detection device, and an output from the one device isinput to the other device, and which generates an AC signal when basedon a level detection result of the level detection device, a level ofthe input signal subject to threshold value operation is a set level,wherein one threshold value operation circuit is made a trigger circuitand the other threshold value operation circuit is made a hold circuit,and an AC signal which is generated based on a logical sum operation onan output from the trigger circuit and an output from the hold circuitbeing input to a modulation device of the hold circuit as a switchingsignal instead of an AC source,

wherein an output signal generated in accordance with the on/offoperation of a start switch comprising first and second contact pointshaving a complimentary relation to each other for on/off switching, isinput to the trigger circuit of the self hold circuit as the inputsignal subject to threshold value operation, and only when the inputsignal subject to threshold value operation is a set level, and theinput signal level to the hold circuit is a set level, is a start signalgenerated from the self hold circuit.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a structural diagram of a first embodiment of a thresholdvalue operation circuit of the present invention;

FIG. 2 is a structural diagram of a second embodiment of a thresholdvalue operation circuit of the present invention;

FIG. 3 is a structural diagram of a third embodiment of a thresholdvalue operation circuit of the present invention;

FIG. 4 is a structural diagram of a fourth embodiment of a thresholdvalue operation circuit of the present invention;

FIG. 5(A) and FIG. 5(B) are specific circuit diagrams of switches in theabove respective embodiments, FIG. 5(A) being an example of where aphotocoupler is used, and FIG. 5(B) being an example of where atransistor is used;

FIG. 6(A) and FIG. 6(B) are respective specific circuit diagrams oflevel detection circuits of the above respective embodiments;

FIG. 7 is a specific circuit diagram of the first embodiment of FIG. 1;

FIG. 8 is a specific circuit diagram of the third embodiment of FIG. 3;

FIG. 9 is a diagram showing a specific circuit example for the casewhere an input signal is an AC signal;

FIG. 10 is a diagram showing a circuit example for where an output froma level detection circuit is transmitted to an output circuit side via aphotocoupler;

FIG. 11 is a circuit diagram of another embodiment of a threshold valueoperation circuit of the present invention;

FIG. 12 is a structural diagram of a threshold value operation circuitof the present invention which uses a Zenner diode;

FIG. 13 is a diagram for explaining problem points for the case where atransistor is used in a switch;

FIG. 14(A) and FIG. 14(B) are circuit diagrams for a construction whichuses a signal with an amplitude of input signal - constant voltage as aswitching signal, FIG. 14(A) being an example applied to the circuit ofFIG. 1, and FIG. 14(B) being an example applied to the circuit of FIG.3;

FIG. 15 is a circuit diagram of a construction which uses a depletiontype transistor;

FIG. 16 is a circuit diagram of a construction which uses a buffercircuit in a modulation device;

FIG. 17 is another circuit diagram of a construction which uses a buffercircuit in a modulation device;

FIG. 18(A) and FIG. 18(B) are another structural diagrams of a thresholdvalue operation circuit of the present invention which uses a Zennerdiode, FIG. 18(A) being an example of a photocoupler applied to thegeneration of an AC output, and FIG. 18(B) being an example of atransistor applied to the generation of an AC output;

FIG. 19 is a circuit diagram with the threshold value operation circuitsof FIG. 12 and FIG. 18 combined;

FIG. 20 is a diagram showing an embodiment of a start signal generatingcircuit to which the threshold value operation circuit of the presentinvention is applied;

FIG. 21 is a diagram showing another structural example of a startsignal generating circuit;

FIG. 22 is a structural diagram of an embodiment of an AND gate circuitof the present invention;

FIG. 23 is a diagram showing a specific circuit example of FIG. 22;

FIG. 24 is a structural diagram of an AND gate circuit for the casewhere an input signal is an AC signal;

FIG. 25 is a diagram showing a specific circuit example of FIG. 24;

FIG. 26 is a structural diagram of an adder circuit in a multiple inputAND gate circuit;

FIG. 27 is a structural diagram of an embodiment of a self hold circuitof the present invention;

FIG. 28 is an operational time chart for the circuit of FIG. 27;

FIG. 29 is a structural diagram of another embodiment of a self holdcircuit;

FIG. 30 is a structural diagram of a start signal generating circuit towhich the self hold circuit of FIG. 29 is applied;

FIG. 31 is a diagram showing a conventional threshold value operationcircuit example; and

FIG. 32 is a diagram showing another example of a conventional thresholdvalue operation circuit.

BEST MODE FOR CARRYING OUT THE INVENTION

As follows is a description of embodiments according to the presentinvention based on the appended drawings.

FIG. 1 shows a circuit structure diagram of a first embodiment of athreshold value operation circuit according to the present invention.

In FIG. 1, a level detection circuit 10 serving as a level detectiondevice, has a threshold value Vt, and when a modulation signal V1 of aninput signal Vi subject to threshold value operation to be describedlater is higher than the threshold value Vt, an output Vo becomes forexample GND level, and when the modulation signal V1 is equal to or lessthan the threshold value Vt, the output Vo becomes a Vcc level.Hereunder the abovementioned input/output relationship is described.However an opposite relationship, that is where the output Vo of thelevel detection circuit becomes the Vcc level when the modulation signalV1 is higher than the threshold value Vt, and the output Vo becomes theGND level when the modulation signal V1 is equal to or less than thethreshold value Vt is also acceptable from an operation point of view.

A switch SWa has one terminal side connected to an input line 11 of theinput signal Vi which constitutes a signal line for an input side andoutput side, and another terminal side connected to a supply line of aconstant voltage Va. Furthermore the arrangement where the switch SWa isswitched on/off in synchronous with a switching signal Sg from aseparately provided external AC power supply (not shown), to modulatethe input signal Vi and generate the modulation signal V1 and input thisto the level detection circuit 10 constitutes a modulation device. Theinput signal Vi is a DC (direct current) signal, which is modulated bythe switch SWa via a resistor R10 disposed in the input line 11, andinput to the level detection circuit 10.

Next is the description of the operation of the first embodiment of FIG.1.

With this circuit, when the switch SWa is switched on, the modulationsignal V1 for input to the level detection circuit 10 becomes a voltageVa. When the switch is switched off, if the input impedance of the leveldetection circuit 10 as seen from GND level is Rc, thenV1=Rc·Vi/(r10+Rc) results (where r10 is the resistance value of theresistor R10).

Here in the case where Va<Vt (Vt: threshold value voltage of the leveldetection circuit 10), then so that the output Vo from the leveldetection circuit 10 becomes an alternating current synchronous with theon/off switching of the switch SWa, V1 when the switch SWa is off mustbe V1=Rc·Vi/(r10+Rc)>Vt, and Vi>(r10+Rc) Vt/Rc must be satisfied.Consequently, in the case where Va<Vt, the threshold value operationcircuit of FIG. 1 has a lower limit threshold value with respect to theinput signal Vi. With the lower limit threshold value VtiL, fromRc·VtiL/(r10+Rc)=Vt, gives VtiL=(r10+Rc)Vt/Rc. Furthermore, when theinput signal Vi is Vi>VtiL, the output Vo from the level detectioncircuit 10 alternates as Vcc/GND, in synchronous with the on/offoperation of the switch SWa due to the switching signal Sg from theexternal AC power supply, so that the output Vo becomes an AC(alternating current) signal (logic value 1) synchronous with theswitching of the switch SWa. On the other hand, in the case whereVi≦VtiL, the output Vo from the level detection circuit 10 does notchange, remaining at a fixed value (as Vcc) even if the switch SWa isswitched on/off, so that the output becomes a DC signal (logic value 0).

Next in the case where Va>Vt, then so that the output Vo from the leveldetection circuit 10 becomes an alternating current in synchronous withthe on/off switching of the switch SWa, V1 when the switch SWa is offmust be V1=Rc·Vi/(r10+Rc)<Vt, and Vi <(r10+Rc)Vt/Rc must be satisfied.Consequently, in the case where Va>Vt, the threshold value operationcircuit of FIG. 1 has an upper limit threshold value with respect to theinput signal Vi. With the upper limit threshold value VtiH, fromRc·VtiH/(r10+Rc)=Vt, gives VtiH=(r10+Rc)Vt/Rc. Furthermore, when theinput signal Vi is Vi<VtiH, the output Vo from the level detectioncircuit 10 alternates as Vcc/GND, in synchronous with the on/offoperation of the switch SWa due to the switching signal Sg from theexternal AC power supply, to give an AC signal. On the other hand, inthe case where Vi≧VtiH, the output Vo from the level detection circuit10 does not change, remaining at a fixed value (as GND) even if theswitch SWa is switched on/off, so that an AC signal is not output.

In the above manner, with the threshold value operation circuit of theconstruction of FIG. 1, by selecting a value for the voltage Va withrespect to the threshold value Vt of the level detection circuit 10, thelower limit threshold value operation and the upper limit thresholdvalue operation can be selected with respect to the input signal Vi.

FIG. 2 is a circuit structure example of another embodiment where it ispossible to select the upper limit/lower limit threshold value operationas in FIG. 1. Components the same as in FIG. 1 are denoted by the samereference symbols and description thereof is omitted.

With the embodiment of FIG. 2, the construction is such that a firstresistor R10 and a switch SWa are disposed in series in an input line 11for an input signal Vi, being the signal line for the input side andoutput side, and a second resistor R11 is provided with one endconnected between the switch SWa and an input terminal of a leveldetection circuit 10, and the other end connected to a supply line of aconstant voltage Va. In this case, a modulation circuit 12 serving as amodulation device is constructed from the first and second resistors R10and R11, and the switch SWa.

Next is a Description of the Operation of the Circuit of FIG. 2

When the switch SWa is off, the input V1 (modulation signal) to thelevel detection circuit 10 becomes V1 (OFF)=Va·Rc/(r11+Rc). When theswitch SWa is on, the input V1 becomes V1(ON)=(r11·Rc·Vi+r10·Rc·Va)/(r10·r11+r10·Rc+r11·Rc). To simplifydescription, making Rc>>r10, r11, then V1 (OFF)Va, and V1(ON)≈(r11·Vi+r10·Va)/(r10+r11).

When Va is determined so that V1 (OFF)Va<Vt, then so that the output Vofrom the level detection circuit 10 becomes an AC signal, the input V1(ON) when the switch SWa is on must be V1(ON)r11·Vi+r10·Va)/(r10+r11)>Vt. Consequently, the circuit of FIG. 2, aswith FIG. 1, becomes a threshold value operation circuit having a lowerlimit threshold value VtiL=Vt+(Vt−Va) r10/r11 with respect to the inputsignal Vi.

On the other hand, when Va is determined so that V1 (OFF)Va>Vt, then sothat the output Vo from the level detection circuit 10 becomes an ACsignal, the input V1 (ON) when the switch SWa is on must be V1(ON)r11·Vi+r10·Va)/(r10+r11)<Vt. In this case, the circuit of FIG. 2becomes a threshold value operation circuit having an upper limitthreshold value VtiH=Vt+(Vt−Va)r10/r11 with respect to the input signalVi.

In the above manner, in the case of the circuit structure of FIG. 2also, the lower limit threshold value operation and the upper limitthreshold value operation can be selected using the value of the voltageVa.

Next, FIG. 3 shows a circuit diagram of a third embodiment. This is anexample of where the threshold value operation circuit has both an upperlimit and lower limit threshold value.

In FIG. 3, with this embodiment the construction is such that a seriescircuit of a switch SWa and a second resistor R11 is connected to aninput line 11 between a first resistor R10 and an input terminal of alevel detection circuit 10.

Next is a Description of the Operation

As with the case of FIG. 2, when the switch SWa of a modulation circuit12 is off, then V1 (OFF)=Rc·Vi/(r10+Rc). When the switch SWa is on, thenV1 (ON)=(r11·Rc Vi+r10·Rc·Va)/(r10·r11+r10·Rc+r11·Rc). Taking Rc>>r10,r11 in the same way as mentioned before, then V1 (OFF)Vi, and V1(ON)(r11·Vi+r10·Va)/(r10+r11).

Furthermore, here Vi takes the range of Vcc<Vi<2Vcc. For example, takingVa=Vcc (that is Va<Vi min), gives V1 (OFF)Vi>V1(ON)(r11·Vi+r10·Vcc)/(r10+r11). Looking at the circuit behaviorcorresponding to the level of the input signal Vi, then with V1(OFF)Vi<Vt, the output Vo from the level detection circuit 10 is fixedat Vcc even if the switch SWa is switched on/off. Moreover, with V1(ON)(r11·Vi+r10·Vcc)/(r10+r11)>Vt, the output Vo from the leveldetection circuit 10 is fixed at GND even if the switch SWa is switchedon/off. The range where the output Vo from the level detection circuit10 becomes a Vcc/GND AC signal synchronous with the on/off switching ofthe switch SWa, is where V1 (OFF)Vi>Vt>V1(ON)(r11·Vi+r10·Vcc)/(r10+r11). Consequently, for the lower limitthreshold value VtiL and the upper limit threshold value VtiH, from V1(OFF)VtiL=Vt, and V1 (ON)(r11·VtiH+r10·Vcc)/(r10+r11)=Vt, theserespectively become VtiL=Vt, and VtiH=Vt+(Vt−Vcc)r10/r11. Therefore thecircuit of FIG. 3 has the function of a window comparator whichgenerates an AC signal output Vo from the level detection circuit 10only when the input signal Vi is within the abovementioned range ofVtiL˜VtiH.

Incidentally, making Va=2Vcc (that is Va>Vi max), gives V1 (OFF)Vi<V1(ON)(r11·Vi+r10·2Vcc)/(r10+r11). In the same way as described before,the output Vo is fixed for either of V1(ON)(r11·Vi+r10+2Vcc)/(r10+r11)<Vt, and V1 (OFF)Vi>Vt. Only in the rangeof V1 (OFF)Vi<Vt<V1 (ON)(r11·Vi+r10·2Vcc)/(r10+r11) does an AC signalappear in the output Vo from the level detection circuit 10, synchronouswith the on/off switching of the switch SWa. For the lower limitthreshold value VtiL and the upper limit threshold value VtiH, from V1(ON)(r11·VtiL+r10·2Vcc)/(r10+r11)=Vt, and V1 (OFF)VtiH=Vt, theserespectively become VtiL=Vt−(2Vcc−Vt)r10/r11, and VtiH=Vt. From theabove, the circuit of FIG. 3 has the function of a window comparatorwhich generates an AC signal output Vo from the level detection circuit10 only when the input signal Vi is within the abovementioned range ofVtiL˜VtiH.

In this manner, with the construction of FIG. 3, there results athreshold value operation circuit having various threshold value rangeswith upper limits and lower limits with respect to the input signal Visubject to threshold value operation.

FIG. 4 shows another circuit structural example having an upper andlower limit threshold value.

With the circuit of FIG. 4, the construction is such that a secondresistor R11 is connected in parallel with a series circuit of a firstresistor R10 and a switch Swa, disposed in series in an input line 11between an input terminal of an input signal Vi and an input terminal ofa level detection circuit 10.

Next is the Description of the Operation.

When the switch SWa is off, then input V1 (OFF)=Vi·Rc/(r10+Rc). When theswitch SWa is on, then input V1 (ON)=Vi·Rc/(r10∥r11+Rc) (herer10∥r11=r10·r11/(r10+r11)). Since V1 (OFF)<V1 (ON), then with V1(OFF)>Vt or V1 (ON)<Vt, the output Vo from the level detection circuit10 is fixed at a constant voltage. Consequently, with the circuit ofFIG. 4 when V1 (OFF)<Vt<V1 (ON), that is, only in the range of(r10∥r11+Rc)Vt/Rc<Vi<(r10+r11)Vt/Rc, does an AC signal appear in theoutput Vo from the level detection circuit 10, synchronous with theswitching operation of the switch SWa. The lower level threshold valueof the circuit of FIG. 4 is VtiL=(r10∥r11+Rc)Vt/Rc, while the upperlimit threshold value is VtiH=(r10+r11)Vt/Rc.

In this way, if the construction is such that the input signal Visubject to threshold value operation, is modulated by a signal Sg froman external AC source, and this modulation signal is level detected,then this give an AC output synchronized with the switching signal ofthe external AC source. Hence, an AC output signal can be obtained witha stabilized frequency and duty ratio, which is not influenced from, forexample input signal level changes or characteristic changes of thecircuit elements. Moreover, the threshold value operation on the inputsignal can be performed as an analogue operation. Furthermore, since theinput signal generation source and the level detection device can havedifferent power sources, this has the effect of increasing the degree offreedom of the layout of the threshold value operation circuit or theinput signal generation source.

FIG. 5(A) and FIG. 5(B) are specific examples of the switch SWaapplicable to the circuits of FIG. 1 through FIG. 4, with FIG. 5(A)being an example using a photocoupler, and FIG. 5(B) being an exampleusing a transistor.

With the switch circuit of FIG. 5(A), a photocoupler PC10 incorporates alight emitting diode PDa and a phototransistor PTa. A switching signalSg from an external AC source 13 is supplied via a resistor Ra to thelight emitting diode PDa.

For example, when the switching signal Sg is an H level, a current flowsin the light emitting diode PDa so that the phototransistor PTa receiveslight and comes on. When the switching signal Sg is an L level, acurrent does not flow in the light emitting diode PDa, and thephototransistor PTa goes off. Consequently, by switching thephototransistor PTa on/off in synchronous with the switching signal Sg,the input signal Vi is modulated, and the modulation signal V1 isgenerated.

With the switching circuit of FIG. 5(B), a switching signal Sg from anAC source 13 is level converted by a capacitor Ca and transmitted to aresistor Ra1. When the switching signal Sg is an H level, a level higherthan at point A in the figure is produced at point B, and a base currentis supplied to the transistor Tr via the resistor Ra1 so that thetransistor Tr comes on. When the switching signal Sg is an L level,point B becomes approximately the same level as point A so that a basecurrent is not supplied and the transistor Tr goes off. In the case ofthis circuit also, by switching the transistor Tr on/off in synchronouswith the switching signal Sg, the input signal Vi is modulated so thatthe modulation signal V1 is produced. A resistor Ra2 is provided inorder to stabilize the off condition of the transistor Tr, while a diodeDa is provided so that when the switching signal Sg is an L level, thecapacitor Ca can be rapidly charged.

In the case of the switching circuits of either (A) or (B) of FIG. 5, inthe case where a fault occurs in the respective elements, the switchingoperation of the phototransistor PTa, and of the transistor Tr isstopped, thus fixing the switching circuit in the on condition or theoff condition.

FIG. 6(A) and FIG. 6(B) are specific circuit examples of a comparatorwhich can be applied to the level detection circuits of FIG. 1 throughFIG. 4.

With the circuit construction of FIG. 6(A), the input threshold valuevoltage Vt is obtained with a Zenner voltage of a Zenner diode ZD asVzd, and an ON voltage between an emitter and base of a transistor Trc1(here an NPN transistor) as Vbe. The limit for the input level for wherethe level of the output Vo from the comparator 10 (level detectioncircuit) changes from Vcc→GND (or from GND→Vcc) is the threshold valuevoltage Vt. With this circuit, the input level which becomes the limitfor the change of the transistor Trc1 from OFF→ON (or from ON→OFF)becomes Vt (that is, the minimum voltage for switching the transistorTrc1 on with an increase in the input V1 level is Vt).

With this circuit, Vt=Vzd+(1+rc1/rc2) Vbe (rc1, rc2 are the resistancevalues of the resistors Rc1, Rc2). When the level of the input V1(modulation signal) is greater than Vt, the transistor Trc1 comes on sothat the output level becomes GND level, while when the input level isequal to or less than Vt, the transistor Trc1 goes off so that theoutput level becomes the Vcc level.

With the circuit construction of FIG. 6(B), when the level of the inputV1 is increased, the threshold value voltage Vt is a minimum voltagewhere the transistor Trc1 (here a PNP transistor) goes off, and when theON voltage between the emitter and base of the transistor Trc1 becomesVbe (>0), then Vt=(1+rc1/rc2)·(Vcc−Vbe). When the level of the input V1is higher than Vt, the transistor Trc1 goes off and the output levelbecomes GND level. When the level of the input V1 is equal to or lessthan Vt, the transistor Trc1 comes on and the output level becomes Vcclevel.

FIG. 7 and FIG. 8 show respective specific circuit constructions for athreshold value operation circuit where the circuit of FIG. 5(A) is usedfor the switch SWa, and the comparator of FIG. 6(B) is used for thelevel detection circuit 10.

FIG. 7 is an example applicable to a threshold value operation circuitof the construction of FIG. 1, and FIG. 8 is an example applicable to athreshold value operation circuit of the construction of FIG. 3. Herewith the circuits of FIG. 7 and FIG. 8, the voltage Va supplied to theother terminal of the switch SWa serves as the power source voltage Vcc.

Since the operation of the circuits of FIG. 7 and FIG. 8 is the same asfor the previous descriptions, except that the voltage Va is replaced bythe power source voltage Vcc, description is here omitted.

With the above respective embodiments, the description has been for thecase where the input signal Vi, which is the subject of the leveldetection, is a direct current. However the input signal subject tolevel detection, may be an AC signal.

To subject an AC signal Si to a threshold value operation, then as shownin FIG. 9, the AC signal subject to threshold value operation, may berectified using for example a voltage doubler rectifying circuit 14 as arectifying circuit, and converted to an DC signal which is then input tothe various threshold value operation circuits described before in FIG.1 through FIG. 4. FIG. 9 shows the case for where the threshold valueoperation circuit of FIG. 8 is used.

In FIG. 9, an output Vi′ from the voltage doubler rectifying circuit 14comprising two diodes D11 and D12 and two capacitors C11 and C12,becomes approximately equal to a DC level where an amplitude Vsi (peakto peak) of the AC signal Si is added to the power source voltage Vcc(that is ViVsi+Vcc). When the DC output Vi′ is within the range from thelower limit threshold value to the upper limit threshold value, an ACsignal synchronous with the switching signal Sg from an external ACsource, is produced in the output Vo.

Incidentally, in the case where a normal two terminal capacitor is usedfor the smoothing capacitor C12 of the voltage doubler rectifyingcircuit 14 rather than the four terminal capacitor shown in the figure,if a disconnection fault occurs in the two terminal capacitor, the ACsignal Si is level converted to the power source level Vcc to appear asthe output Vi′. In this case, even if the switch SWa is not switched, inthe case where the AC signal appearing as the output Vi′ spans thethreshold value of the level detection circuit 10, an AC signal isproduced in the output Vo from the level detection circuit 10. In orderto avoid such a situation, a four terminal capacitor as shown in thefigure may be used in the smoothing capacitor C12. In the case where adisconnection fault occurs in the four terminal capacitor C12, theoutput Vi′ becomes approximately Vcc level, and since the lower limitthreshold value of the level detection circuit 10 is made greater thanthis level, the output Vo from the level detection circuit 10 becomes aDC signal (logic value 0) irrespective of the switching operation of theswitch SWa.

As shown in FIG. 10, the output Vo from the threshold value operationcircuit may be supplied to a photocoupler PC11, and transmitted to theoutput circuit side via the photocoupler PC11.

With such a construction, it becomes possible to use respectiveindependent power supplies for the output circuit side power supply andthe threshold value operation circuit side power supply. Hence thethreshold value operation circuit can be disposed at a position awayfrom the output circuit. This not only increases the degree of freedomof the layout for the input signal generating source and the thresholdvalue operation circuit, but also increases the degree of freedom forthe layout of the output circuit.

Next is the description of a circuit example for performing leveldetection using only the current supplied from the input signal.

In FIG. 11, this circuit is for level detecting an input signal Vi, withthe input signal Vi as the power source for the level detection circuit10. The input signal Vi is supplied as the power source for the leveldetection circuit 10, and on the other hand, this is modulated by aswitching signal Sg from an external AC source using a switch SWa with aphotocoupler, and the level detected by the level detection circuit 10.When the input signal Vi satisfies a set level, a light emitting diodeof a photocoupler PC11 is excited in synchronous with the switchingsignal Sg so that an AC signal is generated via the photocoupler PC11.When the input signal Vi does not satisfy the set level, an AC signal isnot generated from the photocoupler PC11. Here the level detectioncircuit 10 has the function of AC—AC conversion, and when the AC inputlevel to the level detection circuit 10 satisfies the set condition,transmits an AC signal.

Next is a description of a circuit example using a zenner diode forlevel detecting.

FIG. 12 shows a structural example of a threshold value operationcircuit for performing an upper limit/lower limit threshold valueoperation using a Zenner diode. Components the same as for theabovementioned respective embodiments are denoted by the same referencesymbols.

In FIG. 12, the photocoupler PC10 of FIG. 5(A) is used in a switch SWa.A level detection circuit 20 serving as a level detection device of thisembodiment comprises a Zenner diode Tz and a photocoupler PC12. When thelevel of the modulation signal V1 is equal to or above the thresholdvalue (in this case the Zenner voltage Vz) an AC output Vo is generatedvia the photocoupler PC12. A resistor R12 is a current decreasingresistor for controlling the input current to the Zenner diode Tz.

Next is a description of the operation.

This circuit is a threshold value operation circuit constructed suchthat when the input signal Vi is Vz<Vi<Vz/α, the output Vo becomes an ACsignal (logic value 1), while when Vi≧Vz/α or Vi≦Vz, the output Vobecomes a DC signal (logic value 0). Here Vz is the Zenner diodevoltage, and α is α=r11/(r10+r11).

In the case where Vz<Vi<Vz/α, when the switch SWa is switched on, themodulation signal V1 which is the input to the level detection circuit20, becomes V1=αVi, and since αVi<Vz, the Zenner diode Tz goes off, anda current does not flow in the photocoupler PC12 which thus goes off.When the switch SWa goes off, V1=Vi results, and since Vi>Vz, the Zennerdiode Tz conducts and a current flows in the photocoupler PC12 whichthus comes on. Consequently, an AC output Vo is generated from the leveldetection circuit 20 in synchronous with the switching operation of theswitch SWa due to a switching signal Sg from an external AC source.

In the case where Vi≧Vz/α, then even if the switch SWa is on, sinceαVi≧Vz the photocoupler PC12 remains on. Moreover, in the case whereVi≦Vz, then even if the switch SWa is off, since Vi≦Vz the photocouplerPC12 remains off. Consequently with the output Vo from the leveldetection circuit 20, only when there is a threshold value range ofVz<Vi<Vz/α, is an AC signal (logic value 1) generated in synchronouswith the switching operation of the switch SWa. With this circuit, ifthe Zenner diode Tz has a short, the photocoupler PC12 remains in the oncondition, irrespective of the on/off switching of the switch SWa.

As specific examples of the switches SWa for the abovementionedrespective threshold value operation circuits, the photocoupler and thetransistor of FIG. 5(A) and FIG. 5(B) have been shown. With thephotocoupler however, the delay time for the on/off switching isgenerally large so that this is not suitable for high speed switching.On the other hand, while the transistor is suitable for high speedswitching, considering the fail-safe characteristics, there are thefollowing problems.

A circuit where the construction of FIG. 5(B) is applied to thethreshold value operation circuit of FIG. 1 is shown in FIG. 13, andthese problems points are described. Here Va=Vcc.

Since the transistor Tr of FIG. 13 is an enhancement type, the on/offthreshold value voltage (voltage between the base and emitter) of thetransistor Tr is positive. The same applies also with a MOSFET.Consequently, a switching signal Sg of a level within the power sourcelimits of amplitude 0−Vcc, output from the AC source 13 is levelconverted to a switching signal Sg′ of a level outside of the powersource limits of amplitude Vcc−2Vcc, by a capacitor Ca and then suppliedto the base of the transistor Tr. The transistor Tr goes off whenSg′=Vcc, and comes on when Sg′=2Vcc. Here the threshold value Vt of thelevel detection circuit 10 is set to the range of Vcc<Vt<2Vcc. When witha fault of the transistor Tr, the switching signal Sg′ is transmitteddirectly to the level detection circuit 10 as shown by the dotted linein the figure, there is the likelihood of an AC output Vo beinggenerated from the level detection circuit 10 in synchronous with theswitching signal Sg′, regardless of the level of the input signal Vi.With the constant voltage Va set lower than the threshold value voltageVt of the level detection circuit 10, if an enhancement type transistoris used in the switch SWa, there is the likelihood of this problemoccurring.

A structural example of a high speed switching device to solve thisproblem is shown below.

FIG. 14(A) and FIG. 14(B) are examples of constructions where theamplitude of the switching signal Sg generated from the AC source is setto a range between the input voltage Vi and a voltage Va of a constantvoltage supply line, FIG. 14(A) being an example applied to the circuitof FIG. 1, and FIG. 14(B) being an example applied to the circuit ofFIG. 3. In FIG. 14(B), the constant voltage Va=Vcc.

In FIG. 14(A) and FIG. 14(B), a switching signal generating circuit 13′serving as an AC source for generating the switching signal Sg, isconstructed so as to supply input signals Vi and Va (Vcc in FIG. 14(B))as the power source, and output a signal of an amplitude of the range Va(Vcc in FIG. 14(B))—Vi as the switching signal Sg.

With such a construction, the transistor Tr comes on when the switchingsignal Sg is Vi, and goes off when this is Va or Vcc, and the modulationsignal Vi is input to the level detection circuit 10. Even if with afault of the transistor Tr, the switching signal Sg is input directly tothe level detection circuit 10, since the input signal Vi is onlytransmitted in each example of FIG. 14(A) and FIG. 14(B), then if theinput signal Vi is lower than the threshold value voltage Vt, an ACoutput is not generated from the level detection circuit 10 so thatthere is no problem. Consequently, when the switch device is constructedwith a transistor, it is possible to prevent the situation where due toa fault of the transistor the switching signal is input directly to thelevel detection circuit so that an AC signal of logic value 1 isgenerated from the level detection circuit by the switching signal,irrespective of the input signal.

As another structural example of a high speed switching device forsolving the above problem, there is a construction where a signal whichdoes not cross over the threshold value Vt of the level detectioncircuit 10 is used for the switching signal Sg.

FIG. 15 shows an example where this construction is applied to thecircuit of FIG. 3. Here Va=Vcc.

In FIG. 15, an n-type depletion type JFET (junction FET) Tr is used inthe switch SWa, with a source connected to a constant voltage line and adrain connected to an input signal line 11. With the n-type depletiontype transistor, the on/off threshold value voltage (the voltage Vtgsbetween the gate and source with an FET) is negative. Consequently, withthis circuit, when the switching signal Sg is larger than Vtgs (<Vcc),the transistor Tr comes on, while when the switching signal Sg issmaller than Vtgs (<Vcc), the transistor Tr goes off. Therefore a signalhaving for example the amplitude of GND-Vcc may be used as the switchingsignal Sg.

That is to say, it is possible for Sg≦Va, and if the threshold valuevoltage Vt of the level detection circuit 10 has the relation Vt>Va, theswitching signal Sg does not cross over the threshold value voltage Vt,so that even if a fault occurs where the switching signal Sg is inputdirectly to the level detection circuit 10, an AC output Vo is notgenerated from the level detection circuit 10. Moreover, with theconstruction of FIG. 15, there is the advantage that the circuit forlevel converting the switching signal Sg to the signal Sg′ as in FIG.5(B) can be omitted.

In the case where there is a set relation of Vt<Va, if a p-typedepletion type transistor is used, since the on/off threshold valuevoltage of the transistor is positive, then Va≦Sg is possible. Similarlyit is possible to have the switching signal Sg not crossing over thethreshold value voltage Vt. Even if a depletion type MOSFET is used, theconstruction can be the same.

As a device for modulating the input signal using the switching signalSg, a photocoupler or a transistor switch device has been shown. Howevera buffer circuit may also be used. A structural example using buffercircuit is shown in FIG. 16.

FIG. 16 is an example applied to the circuit of FIG. 3.

In FIG. 16, with a modulation circuit 12′ of this embodiment, a buffercircuit B is provided instead of the switch Swa in the modulationcircuit 12 of FIG. 3. The buffer circuit B takes the input of theswitching signal Sg and outputs a switching signal Sg′ of an amplitudeof Va1−Va2 (Va1>Va2) and supplies this to a resistor R11.

Next is a description of the operation.

With the input V1 to the level detection circuit 10, when Sg′=Va1, thisgives V1 (Va1)=(Vi/r10+Va1/r11)×(r10∥r11∥Rc), while when Sg′=Va2, thisgives V1 (Va2)=(Vi/r10+Va2/r11)×(r10∥r11∥Rc). Herer10∥r11∥Rc=1/(1/r10+1/r11+1Rc). Since Va1>Va2, then V1 (Va1)>V1 (Va2),and when with respect to the threshold value voltage Vt of the leveldetection circuit 10, there is the relation Vt>V1 (Va1) or V1 (Va2)>Vt,the output Vo from the level detection circuit 10 is fixed at a constantvoltage. Consequently, with this circuit, when V1 (Va1)>Vt>V1 (Va2),that is, only in the range ofr10×(Vt/(r10μr11∥Rc)−Va1/r11)<Vi<r10<r10×(Vt/(r10∥r11∥Rc)−Va2/r11), isan AC signal synchronous with the switching signal Sg′ generated in theoutput Vo from the level detection circuit 10. That is to say, thecircuit of FIG. 16 has a lower limit threshold valueVtiL=r10×(Vt/(r10∥r11∥Rc)−Va1/r11), and an upper limit threshold valueVtiH=r10×(Vt/(r10∥r11∥Rc)−Va2/r11), with respect to the input signal Vi.

Here if Rc>>r10, r11, and moreover Vi>Vcc, Va1=Vcc, Va2=0 V (GND), thenthe lower limit threshold value is VtiL=(1+r10/r11)×Vt−Vcc×r10/r11, andthe upper limit threshold value is VtiH=(1+r10/r11)×Vt. Here the outputimpedance of the buffer circuit B is made zero.

Needless to say, it does not matter if the construction is such that theswitching signal Sg is supplied as is to the resistor R11, withoutconversion to the switching signal Sg′.

With the construction where the switching signal Sg is supplied as is tothe resistor R11, a construction is also considered where the switchingsignal is made proportional to the input signal Vi. With FIG. 17, thereis a signal of an amplitude of KVi−Va2 serving as the switching signalSg′, being the output from the buffer circuit B. Here KVi (K is aproportional constant) is proportional to the input signal Vi, withKVi>Va2.

The operation is the same as with FIG. 16, with Va1 in the circuit ofFIG. 16 replaced by KVi.

That is to say, with the modulation input V1 to the level detectioncircuit 10, when Sg′=KVi, V1 (KVi)=(Vi/r10+KVi/r11)×(r10∥r11∥Rc), whilewhen Sg′=Va2, this gives V1 (Va2)=(Vi/r10+Va2/r11)×(r10∥r11∥Rc). SinceKVi>Va2, then V1 (KVi)>V1 (Va2), and when with respect to the thresholdvalue voltage Vt of the level detection circuit 10, there is therelation Vt>V1 (KVi) or V1 (Va2)>Vt, the output Vo from the leveldetection circuit 10 is fixed at a constant voltage. With this circuit,when V1 (KVi)>Vt>V1 (Va2), that is, only in the range ofVt/((1/r10+k/r11)×(r10∥r11∥Rc))<Vi<r10×(Vt/(r10∥r11∥Rc)−Va2/r11), is anAC signal synchronous with the switching signal Sg′ generated in theoutput Vo from the level detection circuit 10. That is to say, this hasa lower limit threshold value VtiL=Vt/((1/r10+k/r11)×(r10∥r11∥Rc)), andan upper limit threshold VtiH=r10×(Vt/(r10∥r11∥Rc)−Va2/r11), withrespect to the input signal Vi.

If Rc>>r10, r11, and Vi, Vt>Vcc, k=1, Va2=Vcc, then the lower limitthreshold value is VtiL=Vt, and the upper limit threshold value isVtiH=(1+r10/r11)×Vt−Vcc×r10/r11. Here the output impedance of the buffercircuit B is made zero.

A description is now given for the case where in the circuit of FIG. 17,r11=0, that is for the case with no resistor R11, applied to the circuitof FIG. 1.

The input V1 to the level detection circuit 10 is KVi when Sg′=KVi, andVa2 when Sg′=Va2. If Vt>Va2, then when Vt>KVi, the output Vo from thelevel detection circuit 10 is fixed at a constant voltage. Consequently,when KVi>Vt, that is, only in the range of Vi>Vt/k is an AC signalsynchronous with the switching signal Sg′ generated in the output Vofrom the level detection circuit 10. That is to say, this has a lowerlimit threshold value VtiL=Vt/k with respect to the input signal Vi. Onthe other hand, if Va2>Vt, then when KVi>Vt, the output Vo from thelevel detection circuit 10 is fixed at a constant voltage. Consequently,when Vt>KVi, that is, only in the range of Vt/k>Vi is an AC signalsynchronous with the switching signal Sg′ generated in the output Vofrom the level detection circuit 10. That is to say, this has an upperlimit threshold value VtiH=Vt/k with respect to the input signal Vi.

In this case, as can be seen from the above equation, the respectivethreshold values are unrelated to the resistor R10, and hence theresistor R10 can be removed.

FIG. 18(A) and FIG. 18(B) show another embodiment of a threshold valueoperation circuit corresponding to a third aspect of the presentinvention. The construction is such that after an input signal Visubject to threshold value operation, is level detected by a Zennerdiode Tz, this is modulated by a modulation device.

In FIG. 18(A) the construction is such that the Zenner diode Tz servingas a level detection device, is inserted in an input line 11 between aninput terminal for the input signal Vi and a first resistor R10. Otherconstruction is the same as that of FIG. 12.

With the circuit of this embodiment, when the input signal Vi isVL_(ED)+Vz<Vi<VL_(ED)/α+Vz, the output Vo becomes an AC signal (logicvalue 1), while when this is the Vi>VL_(ED)/α+Vz or Vi<VL_(ED)+Vz, theoutput Vo becomes a DC signal (logic value 0). Here Vz is the Zennervoltage of the Zenner diode Tz, VL_(ED) is the falling voltage of thelight emitting diode of a photocoupler PC12 serving as the AC signalgenerating device, and α=r11/(r10+r11).

In the case where VL_(ED)+Vz<VL_(ED)/α+Vz, then when the switch SWa ison, the modulation signal V1 input to the light emitting diode of thephotocoupler PC12 becomes V1=α(Vi−Vz), and since a (Vi−Vz)<VL_(ED), acurrent does not flow in the photocoupler PC12 which thus goes off. Ifthe switch SWa is off, then V1=Vi−Vz, and since Vi−Vz>VL_(ED), a currentflows in the photocoupler PC12 which thus comes on.

On the other hand, in the case where Vi≧VL_(ED)/α+Vz, then even if theswitch SWa is on, since a (Vi−Vz)≧VL_(ED), the photocoupler PC12 remainson. Moreover, in the case where Vi≦VL_(ED)+Vz, then even if the switchSWa is off, since Vi−Vz≦VL_(ED) the photocoupler PC12 remains off.

Consequently, only when the output Vo from the photocoupler PC12 iswithin a threshold value range of VL_(ED)+Vz<Vi<VL_(ED)/α+Vz is an ACsignal (logic value 1) generated in synchronous with the switchingoperation of the switch SWa. With this circuit also, if the Zenner diodeTz is short-circuited, the photocoupler PC12 remains in the on conditionirrespective of the on/off switching of the switch SWa.

FIG. 18(B) is an example of where a transistor Tr12, as surrounded bythe dotted line in the figure, is used instead of the photocoupler PC12,as the AC signal generating device. In this case, if the voltage betweenthe base and emitter when the transistor Tr12 is on is made Vbe, and thefalling voltage VL_(ED) of the light emitting diode of the photocouplerPC12 is replaced with Vbe, this constitutes the same arrangement as thecircuit of FIG. 18(A) (provided that the current passed by the resistorR13 can be ignored).

The following description is given for the case of the circuitconstruction using the photocoupler PC12. However also in the case wherethe transistor Tr12 is used, if in the circuits of FIG. 19 through FIG.21, VL_(ED) is replaced with Vbe, then this constitutes the samearrangement as the circuit of FIG. 18(A).

In the case of the circuits of FIG. 12 and FIG. 18, also when the Zennerdiode Tz is short-circuited, if the input signal Vi is within the rangeof αVi<VL_(ED), and Vi>VL_(ED), there is the problem that thephotocoupler PC 12 switches on/off following the switching operation ofthe switch SWa.

FIG. 19 shows a structural example of a threshold value operationcircuit corresponding to claim 21 for solving the above problem.

In FIG. 19, this threshold value operation circuit is a combination ofthe circuit of FIG. 12 and the circuit of FIG. 18. That is to say, theconstruction is such that the photocoupler PC12 of the threshold valueoperation circuit of FIG. 18 is used as the switch SWa of FIG. 12. Herein the figure, R10′, R11′, R12′ correspond to the respective resistorsR10, R11, R12 of FIG. 12, and Tz′, PC12′ respectively correspond to theZenner diode Tz and the photocoupler PC12 in the level detection circuit20 of FIG. 12. Consequently, the Zenner diodes Tz, Tz′ correspond to thefirst and second Zenner diodes, the switch SWa corresponds to theswitching device, the photocoupler PC12 corresponds to the firstphotocoupler, and the photocoupler PC12′ corresponds to the secondphotocoupler.

With such a construction, the input signal Vi subject to threshold valueoperation, is subjected to a threshold value operation by the firstZenner diode Tz, and if this is the set level, an AC signal is generatedfrom the first photocoupler PC12. Then, with the AC signal from thefirst photocoupler PC12 as a switching signal, the input signal Vi ismodulated and input to the second Zenner diode Tz′ and level detected.If the level detection result is such that the input modulation signalis the set level, an AC signal (logic value 1) is generated from thesecond photocoupler PC12′. That is to say, the lower limit thresholdvalue is set by the first Zenner diode Tz, the upper limit thresholdvalue is set by the second Zenner diode Tz′, and the upper limitthreshold value operation is executed with the output result of thelower limit threshold value operation as the modulation signal.

With such a construction, since the lower limit and upper limit of therespective threshold values can be independently set, then in the casewhere the Zenner diode Tz or Tz′ is short-circuited, generation of an ACsignal (logic value 1) from the second photocoupler PC12 can beprevented.

In FIG. 19, in the case where an input signal level exists between thelower limit threshold value Vz and the upper limit threshold valueVz′/α′ (here α′=r′11/(r10′+r11′)), the passing current of the lightemitting diode of the photocoupler PC12′ is approximately (Vz′−Vz)/r12′at a minimum. In the case where the Zenner diodes Tz and Tz′ both short,the range of the input level where a possibility exists for theoccurrence of an AC signal in the output of the photocoupler PC12′ is:

VL_(ED)<Vi<min (VL_(ED)/α, VL_(ED)α′)  (1)

where α=r11/(r10+r11).

At this time, the passing current of the light emitting diode of thephotocoupler PC12′ is (VL_(ED)/r12′)·(1−α)/α, or(VL_(ED)/r12′)·(1−α′)/α.

If Vz′−Vz is set in sufficiently large proportion to VL_(ED)×min((1−α′)/α, (1−α)/α), then even in the case where the Zenner diodes Tzand Tz′ both short and the input voltage is kept within the narrow rangedefined by the above equation (1), the generation of an output currentof the photocoupler PC12′ can be suppressed.

Next an application example for the threshold value operation circuit ofthe present invention is described.

FIG. 20 shows an example of a start signal generating circuit for amachine, having a level detection function which uses the abovementioned threshold value operation circuit.

In FIG. 20, with this circuit, the construction is such that a startsignal is generated while verifying the normalcy of a start switch 31having a first contact point 31 a and a second contact point 31 b with acomplementary relationship to each other. That is to say, a back-checkas to whether or not the second contact point 31 b comes on after thefirst contact point 31 a comes on is performed, and when the secondcontact point 31 b comes on, the start signal is generated.

When the start switch 31 is pressed so that the first contact point 31 acomes on, a capacitor C31 is charged via a diode D31. After this, whenthe start switch 31 is returned and the second contact point 31 b comeson, the capacitor C31 discharges. The construction is such that thisdischarge signal is input to the threshold value operation circuit ofthe present invention as the input signal Vi subject to threshold valueoperation, and subjected to threshold value operation, and only whenthis is the set level, is an AC (logic value 1) start signal Vogenerated from the photocoupler PC12 of the threshold value operationcircuit. With the present embodiment, the threshold value operationcircuit using the Zenner diode Tz shown FIG. 12 is applied, howeverneedless to say other threshold value operation circuits are suitable.

With the present embodiment, the input voltage Vi at the time ofdischarge is a level of 2Vcc, while at the time of charging this is Vcc.Consequently, so that an AC signal is generated when Vi=2Vcc, and an ACsignal is not generated when Vi=Vcc, then α(=r11/(r10+r11)) and theZenner voltage Vz are set so as to satisfy Vcc<Vz<2Vcc, and 2Vcc/α<Vz.

The charge stored in the capacitor C31 is discharged by the diode D32after the first contact point 31 agoes off.

In FIG. 20, in the case where the Zenner diode Tz is short-circuited,then in the course of discharging the charge which has been charged tothe capacitor C31, there is a period where the signal level voltagedivided by the resistors R10 and R11 is within a range VL_(ED) andVL_(ED)/α (α=r11/(r10+r11)). However the generation time for this isextremely short compared to the case where the Zenner diode Tz isnormal.

FIG. 21 shows another example of this type of start signal generatingcircuit for a machine.

The circuit of FIG. 21 is one where the threshold value operationcircuit of FIG. 19 is applied.

In FIG. 21, the threshold value operation circuits 101 and 102 are thoseshown in FIG. 19. The threshold value operation circuit 101 performs athreshold value operation with respect to the power source voltage Vcc,with the power source voltage Vcc as the input signal. The outputtherefrom is generated as an AC switching signal Sg′ for the thresholdvalue operation circuit 102. The threshold value operation circuit 102,with the signal Vi generated when an operation button 103 is switchedon, as the input signal subject to threshold value operation, performs athreshold value operation with respect to this signal Vi and generatesan output signal Vo as the final start signal. A capacitor C101, anddiodes D101, D102 correspond to the capacitor C31 and the diodes D31,D32 in FIG. 20.

With such a construction, if the power source voltage Vcc satisfies aset level, the threshold value operation circuit 101 generates an ACoutput synchronized with an external AC switching signal Sg, and inputsthe AC switching signal Sg′ to the threshold value operation circuit102. Then, if the signal Vi when the operation button 103 is switched onsatisfies the set level, an AC output signal Vo synchronized with theswitching signal Sg′ is generated from the threshold value operationcircuit 102. The level of the signal Vi increases from Vcc to 2Vcc whenthe operation button 103 is pressed on. The lower limit threshold valueof the threshold value operation circuit 102 is set between Vcc and2Vcc.

Here if the power source voltage Vcc itself increases, the level of thesignal Vi exceeds the threshold value of the threshold value operationcircuit 102. However since in the case where the increased power sourcevoltage Vcc level does not satisfy the set level of the threshold valueoperation circuit 101, the switching signal Sg′ is not generated fromthe threshold value operation circuit 101, an AC output signal Vo is notgenerated from the threshold value operation circuit 102. Consequently,only when the power source voltage Vcc is at a normal level is theoutput signal Vo generated.

Next is a description of an AND gate circuit which uses the thresholdvalue operation circuit of the present invention.

FIG. 22 shows a structural example of an AND gate circuit which uses thethreshold value operation circuit of FIG. 3. Needless to say, an ANDgate circuit can be constructed similarly from the other threshold valueoperation circuits.

In FIG. 22, the AND gate circuit of this embodiment comprises a firstthreshold value operation circuit comprising resistors R10, R11, aswitch SWa1, and a level detection circuit 10, and a second thresholdvalue operation circuit comprising resistances R20, R21, a switch Swa2,and a level detection circuit 10′. The construction is such that anoutput Vo from the first threshold value operation circuit 10 is made aswitching signal for the switch SWa2 of the second threshold valueoperation circuit 10′. With this AND gate circuit, when the DC inputsignals Vi1 and Vi2 are both within respectively predetermined levels,an AC signal (corresponding to logic value 1) is generated as an outputVo2 from the second threshold value operation circuit.

Hereunder is a description of the operation.

The switch SWa1 of the first threshold value operation circuit switchesin synchronous with a switching signal Sg from a separately provided ACsource.

When the input Vi1 is within a range from a lower limit threshold valueVt1L to an upper limit threshold value Vt1H determined for example byresistors R10, R11, Va and a threshold value Vt1 of the level detectioncircuit 10, an AC signal (corresponding to logic value 1) is generatedas the output Vo1 from the first threshold value operation circuit. ThisAC output signal Vo1 is supplied to the switch SWa2 of the next stagesecond threshold value operation circuit, and the switch SWa2 switchesin synchronous with the signal Vo1. When the input signal Vi2 is withina range from a lower limit threshold value Vt2L to an upper limitthreshold value Vt2H determined for example by resistors R20, R21, Vaand a threshold value Vt2 of the level detection circuit 10′, an ACsignal (corresponding logic value 1) is generated as the output Vo2.

If for example the input Vi1 is less than the lower limit thresholdvalue Vt1L or greater than the upper limit threshold value Vt1H, theoutput Vo1 from the first threshold value operation circuit becomes a DCsignal (corresponding to logic value 0), and an AC signal is notgenerated. Consequently, the switch SWa2 of the second threshold valueoperation circuit does not switch, and is fixed in the on condition orthe off condition. Therefore, even if the input signal Vi2 is within thethreshold value range, the output Vo2 from the second threshold valueoperation circuit becomes a DC signal (that is, logic value 0) and an ACsignal is not generated. Moreover, also when the input signal Vi1 iswithin the threshold value range but the input signal Vi2 is outside ofthe threshold value range, an AC signal is not generated in the outputVo2 of the second threshold value operation circuit.

In this way, the circuit of FIG. 22 is an AND gate circuit in that onlywhen the input signals Vi1 and Vi2 are both within their respectivelydetermined threshold value ranges (i.e., both logic value 1) is an ACsignal output (a logic value 1 generated).

If the construction is such that a threshold value operation circuit isfurther connected in cascade so that a switch SWa of a next stagethreshold value operation circuit is driven by an output Vo from aprevious stage threshold value operation circuit, then a multiple inputAND gate circuit can be constructed, enabling multiple input ANDoperations (logical product operations) to be performed.

FIG. 23 is a specific circuit example of a construction where thephotocoupler of FIG. 5(A) is used as the first and second respectiveswitches SWa1 and SWa2, and the comparator of FIG. 6(B) is used as leveldetection circuits 10 and 10′ in the AND gate circuit of FIG. 22. Inthis case, the construction is such that a light emitting diode of thenext stage switch SWa2 is connected via a resistor Ra1 to the output Vo1of the first threshold value operation circuit, and by driving the lightemitting diode with the output Vo1, a phototransistor of the switch SWa2is switched.

Here since the operation of the respective threshold value operationcircuits is the same as described beforehand, description will beomitted.

FIG. 24 shows a structural example of an AND gate circuit for the casewhere the input signals are both AC signals, illustrating a structuralexample of where the threshold value operation circuit of FIG. 3 isused. Needless to say, this can also be similarly constructed from theother threshold value operation circuits.

In FIG. 24, the AND gate circuit of this embodiment provides an addercircuit 40 comprising first and second voltage doubler rectifyingcircuits 40A and 40B, each comprising two capacitors and two diodes, forrespectively voltage doubler rectifying AC input signals Si1 and Si2.The adder circuit 40 adds the rectified output Vi1 from the firstvoltage doubler rectifying circuit 40A which rectifies the AC signalSi1, to the rectified output Vi2 from the second voltage doublerrectifying circuit 40B which rectifies the AC signal Si2, and generatesan addition output of Vi1+Vi2. The construction is such that thisaddition output (Vi1+Vi2) is input to the threshold value operationcircuit as the input signal Vi, and subjected to a threshold valueoperation by the threshold value operation circuit.

Hereunder is a description of the operation.

The addition output level Vi of the adder circuit 40, with the logicvalue 1 amplitudes (peak—peak) of the AC input signals Si1 and Si2respectively as Vi1, Vi2, becomes ViVi1+Vi2+Vcc. If the lower limitthreshold value VtL is set in the range Vi1+Vcc,Vi2+Vcc<VtL<Vi1+Vi2+Vcc, then only when the AC input signals Si1, Si2are both input at a level of logic value 1 is an AC signal (logicvalue 1) output.

FIG. 25 shows a specific circuit structural example for where in the ANDgate circuit of FIG. 24, the photocoupler of FIG. 5(A) is used in theswitch SWa of the threshold value operation circuit, and the comparatorof FIG. 6(B) is used in the level detection circuit 10. Since theoperation of the threshold value operation circuit is the same asmentioned before, description is omitted.

In the case where, as with the circuit shown as an example in FIG. 25, athreshold value operation circuit having upper and lower limit thresholdvalues is used, if the lower limit threshold value VtL and upper limitthreshold value VtH are set for example as Vcc<VtL<Vi1+Vcc,Vi2+Vcc<VtH<Vi1+Vi2+Vcc, then it is also possible for an AC output to begenerated from the threshold value operation circuit only when eitherone of the AC input signals Si1 or Si2 is input (logic value 1).

In FIG. 24 an AND gate circuit construction for the case of two inputsis shown. To have an n input AND gate circuit, an adder circuit can beconstructed from n voltage doubler rectifying circuits, and the lowerlimit threshold value of the threshold value operation circuit setbetween a rectified addition level for when n AC input signals are allat a logic value 1level, and a rectified addition level for when one ofthe n AC input signals is at a logic value 0 level.

FIG. 26 shows a structural example of an adder circuit 50 for when n=8.

In FIG. 26, the addition output Vi (ΣVin (n=1˜8)) from the adder circuit50 is input to a threshold value operation circuit of the presentinvention as the input signal. In this case, if the rectified outputfrom one rectifying circuit is ViJ, the lower limit threshold ViL of thethreshold value operation circuit is set to the range 7Vij+Vcc<ViL<8Vij+Vcc. In this way, only when all of the input AC signals Si1˜Si8 areat a level of logic value 1 is an AC signal generated from the thresholdvalue operation circuit.

The construction of the adder circuit by such rectifying circuits isknown for example from International Patent Publication WO93/23772 orfrom “Application of Fail-safe Multi-Valued Logic to Optical SensorScanning Circuit”, (National Meeting of the Japanese Institute ofElectrical Engineers, 699, 1993).

Next is a description of a self hold circuit which uses the thresholdvalue operation circuit of the present invention.

FIG. 27 shows a specific circuit structural example of a self holdcircuit of the invention.

In FIG. 27, with the self hold circuit of this embodiment, theconstruction is such that the output Vo from the AND gate circuit shownin FIG. 25 is rectified by a voltage doubler rectifying circuit 60, andthe resultant rectified output Vf is fed back to an output terminal of afirst voltage doubler rectifying circuit 40A of an adder circuit 40. Aninput terminal of a second voltage doubler rectifying circuit 40B ismade a reset input terminal (or hold input terminal) and an inputterminal of the first voltage doubler rectifying circuit 40A is made atrigger input terminal. Components the same as for the embodiment ofFIG. 25 are denoted by the same reference symbols. The voltage doublerrectifying circuit 60 is constructed the same as the first and secondvoltage doubler rectifying circuits 40A and 40B.

Next is a description of the operation based on the time chart shown inFIG. 28.

Here, at a logic value 1, the input signal Si is an AC signal with anamplitude (peak—peak) Vsi, and the input signal Ti is an AC signal withan amplitude (peak—peak) Vti, while at logic value 0, both become DCsignals.

When an AC input signal Si is not input to the reset input terminal (atthe time of logic value 0), the input Vi being the rectified additionoutput is approximately Vcc. When at the time t1, the AC input signal Siis input (logic value 1), the input Vi to the threshold value operationcircuit becomes approximately Vsi+Vcc. Here if the lower limit thresholdvalue VtL of the threshold value operation circuit is set to Vsi+Vcc,Vti+Vcc<VtL<Vsi+Vti+Vcc, Vsi+Voi+Vcc, then in the time from t1˜t2, theoutput Vo from the threshold value operation circuit is a DC signal(logic value 0). When at time t2, an AC input signal Ti is input to thetrigger input terminal (logic value 1), the input Vi become Vsi+Vti+Vcc,and exceeds the lower limit threshold value VtL. Therefore the output Vofrom the threshold value operation circuit becomes logic value 1, and anAC signal (amplitude Voi (peak—peak)Vcc) is generated. This AC signal isinput to the voltage doubler rectifying circuit 60 and rectified, and arectified output Vf=Voi+Vcc is produced, and fed back by an ORconnection to the output terminal of the first voltage doublerrectifying circuit 40A of the adder circuit 40. At time t3, even if theAC signal Ti of the trigger input terminal disappears, since therectified signal Vf of the output Vo is being fed back, the input Vi ismaintained at the condition Vsi+Voi+Vcc>VtL, so that the output Vo=1 (ACsignal) continues. When at time t4 the AC input signal Si disappears,the input Vi becomes Voi+Vcc<VtL, so that the output Vo becomes a DCsignal (logic value 0).

The threshold value operation circuit applied to the self hold circuitof FIG. 27 is not limited to this construction, and needless to say maybe the other threshold value operation circuits of the presentinvention.

FIG. 29 shows another structural example of a self hold circuit to whichthe threshold value operation circuit of FIG. 11 is applied.

In FIG. 29, a trigger circuit 111 and a hold circuit 112 both have thesame construction as the threshold value operation circuit of FIG. 11.The trigger circuit 111 modulates the input signal Vi1 by means of an ACswitching signal Sg from outside, to perform threshold value operation.The hold circuit 112 modulates the input signal Vi2 with an AC signalgenerated as a result of the threshold value operation of the triggercircuit 111 as a switching signal Sg′, to thereby perform thresholdvalue operation. The output from the hold circuit 112 supplies thecurrent for generating the switching signal Sg′. Numerals 113 and 114 inthe figure denote the before mentioned voltage doubler rectifyingcircuits, and PC denotes a photocoupler which switches the rectifiedoutput from the voltage doubler rectifying circuit 113 in synchronouswith the external AC switching signal Sg, to generate the switchingsignal Sg′.

With the operation of this self hold circuit, when the input signal Vi1satisfies a set level, the trigger circuit 111 generates an outputsignal Vo1. This output signal Vo1 is rectified by the voltage doublerrectifying circuit 113, and a switching signal Sg′ is generated via thephotocoupler PC by the switching signal Sg. The input signal Vi2 of thehold circuit 112 is modulated by this switching signal Sg′, and when theinput signal Vi2 satisfies the set level, an output signal Vo2 isgenerated from the hold circuit 112. This output signal Vo2 is rectifiedby the voltage doubler rectifying circuit 114 and output to the outsideas an output signal V′o2, and is also supplied to the output side of thevoltage doubler rectifying circuit 113 to supply a current forgenerating the switching signal Sg′. Consequently, after the switchingsignal Sg′ is generated with generation of the input signal Vi1, and theoutput signal Vo2 is generated with the input of the input signal Vi2,then even if the input signal Vi1 being the trigger input disappears,then provided that the input signal Vi2 being the hold input does notdisappear, the generation of the output signal Vo2 continues. Afterthis, when the input signal Vi2 disappears, the output signal Vo2disappears, so that the switching signal Sg′ stops, and as long as theinput signal Vi1 being the trigger input is not again input, theswitching signal Sg′ is not generated, and output signal Vo2 is notgenerated.

FIG. 30 shows a structural example for the case where the self holdcircuit of FIG. 29 is applied to the start signal generating circuit ofFIG. 20.

In FIG. 30, a trigger circuit 111′ in the start signal generatingcircuit corresponds to a part in FIG. 20 where the start switch 31 isremoved.

With such a construction, when the start switch 31 is switched off(first contact point 31 a is off, second contact point 31 b is on) afterbeing on (first contact point 31 a is on), then with the power sourcevoltage Vcc normal, an output signal Vo1 is generated from the triggercircuit 111′, and a switching signal Sg′ is generated. Then, if theinput signal Vi of the hold circuit 112 is a set level, an output signalVo2 is generated from the hold circuit 112, and when an output signalV′o2 is generated from the voltage doubler rectifying circuit 114, theswitching signal Sg′ is self held by this output signal Vo2′.

INDUSTRIAL APPLICABILITY

With the present invention, analogue type threshold value operation ispossible with respect to the input signal, and an AC signal havingstabilized frequency and duty ratio can be generated. Therefore, theinvention can be used for fail-safe power sources and like, and henceindustrial applicability is great.

What is claimed is:
 1. A threshold value operation circuit comprising:modulation means for modulating an input signal by means of a switchingsignal from an AC (alternating current) source; and level detectionmeans for level detecting the modulated input signal by comparing themodulated input signal with a previously set threshold value, andgenerating an AC signal when, based on a level detection result of saidlevel detection means, a level of said modulated input signal is a setlevel.
 2. A threshold value operation circuit according to claim 1,wherein said level detection means includes an AC signal generatingmeans which includes a photocoupler.
 3. A threshold value operationcircuit according to claim 1, wherein said modulation means switches aswitch means by said switching signal from said AC source, to modulatesaid input signal.
 4. A threshold value operation circuit according toclaim 3, wherein with said modulation means, said switch means isdisposed between a signal line between an input side and an output side,and a constant voltage supply line.
 5. A threshold value operationcircuit according to claim 4, wherein said modulation means incorporatesa first resistor disposed in said signal line between the input side anda contact point of said switch means, and a second resistor disposedbetween the switch contact point of said signal line and said switchmeans.
 6. A threshold value operation circuit according to claim 3,wherein said modulation means incorporates a first resistor and saidswitch means disposed in series in a signal line between an input sideand an output side, and a second resistor with one end connected to thesignal line between said switch means and output side, and the other endconnected to a constant voltage supply line.
 7. A threshold valueoperation circuit according to claim 3, wherein said modulation meansincorporates a first resistor and said switch means disposed in seriesin a signal line between an input side and an output side, and a secondresistor connected in parallel with a series circuit of said firstresistor and switch means.
 8. A threshold value operation circuitaccording to claim 3, wherein said switch means is a photocoupler whichis switched by input of the switching signal from said AC source.
 9. Athreshold value operation circuit according to claim 3, wherein saidswitch means is a transistor which is switched by input of the switchingsignal from said AC source.
 10. A threshold value operation circuitaccording to claim 9, wherein the switching signal input to saidtransistor has an amplitude of a range between said input signal subjectto threshold value operation and the voltage of a constant voltagesupply line.
 11. A threshold value operation circuit according to claim9, wherein the switching signal input to said transistor is a signalwhich does not cross over a threshold value of said level detectionmeans.
 12. A threshold value operation circuit according to claim 1,wherein said modulation means incorporates a buffer circuit which takesthe input of said AC source switching signal, and supplies this to aninput line to which said input signal is applied.
 13. A threshold valueoperation circuit according to claim 12, wherein said buffer circuitsupplies a signal with an amplitude proportional to said input signal tosaid input line.
 14. A threshold value operation circuit according toclaim 1, wherein said level detection means is a comparator.
 15. Athreshold value operation circuit according to claim 14, wherein an ACoutput from said comparator is transmitted to an output circuit side viaa photocoupler.
 16. A threshold value operation circuit according toclaim 1, wherein said level detection means incorporates a Zenner diodeand a photocoupler for generating the AC signal which is switched by anoutput from said Zenner diode.
 17. A threshold value operation circuitaccording to claim 1, wherein said level detection means level detectswith only the current supplied by said input signal.
 18. A thresholdvalue operation circuit according to claim 17, wherein said leveldetection means is a Zenner diode.
 19. A threshold value operationcircuit comprising: a first Zenner diode for level detecting an inputsignal subject to threshold value operation; a switch means which isswitched by a switching signal from an AC source, for modulating anoutput signal from said first Zenner diode; a first photocoupler whichis switched by input of a modulation signal modulated by said switchmeans, for modulating said input signal subject to threshold valueoperation; a second Zenner diode for level detecting a modulation signalof said input signal subject to threshold value operation which has beenmodulated by the switching operation of said first photocoupler, and asecond photocoupler which is switched by an output signal from saidsecond Zenner diode, for generating an AC signal, wherein when the levelof said input signal subject to threshold value operation is a setlevel, the AC signal is generated from said second photocoupler.
 20. AnAND gate circuit comprising a plurality of threshold value operationcircuits connected in cascade each of which comprises: modulation meansfor modulating an input signal by means of a switching signal from an AC(alternating current) source; and level detection means for leveldetecting the modulated input signal by comparing the modulated inputsignal with a previously set threshold value, and generating an ACsignal when, based on a level detection result of said level detectionmeans, a level of said modulated input signal is a set level, whereinthe AC output from a former stage threshold value operation circuit isinput as the switching signal to the modulation means of a latter stagethreshold value operation circuit, instead of the AC source.
 21. An ANDgate circuit comprising: a threshold value operation circuit whichcomprises: modulation means for modulating an input signal by means of aswitching signal from an AC (alternating current) source; and leveldetection means for level detecting the modulated input signal bycomparing the modulated input signal with a previously set thresholdvalue, and generating an AC signal when based on a level detectionresult of said level detection means, a level of said modulated inputsignal is a set level; and an adder circuit comprising a plurality ofrectifying circuits for respectively rectifying AC input signals, whichsequentially adds a rectified output from a first stage rectifyingcircuit to a rectified output from a next stage rectifying circuit, andoutputs the added values for all of the AC input signals from a finalstage rectifying circuit, wherein the addition output from said addercircuit is supplied as the input signal subject to threshold valueoperation of said threshold value operation circuit.
 22. A self holdcircuit comprising two threshold value operation circuits each of whichcomprises: modulation means for modulating an input signal by means of aswitching signal from an AC (alternating current) source; and leveldetection means for level detecting the modulated input signal bycomparing the modulated input signal with a previously set thresholdvalue, and generating an AC signal when based on a level detectionresult of said level detection means, a level of said modulated inputsignal subject to threshold value operation is a set level, wherein onethreshold value operation circuit is made a trigger circuit and theother threshold value operation circuit is made a hold circuit, and anAC signal which is generated based on a logical sum operation on anoutput from said trigger circuit and an output from said hold circuit isinput to a modulation means of said hold circuit as the switching signalinstead of the AC source.
 23. A self hold circuit comprising: athreshold value operation circuit which comprises: modulation means formodulating an input signal by means of a switching signal from an AC(alternating current) source; and level detection means for leveldetecting the modulated input signal by comparing the modulated inputsignal with a previously set threshold value, and generating an ACsignal when based on a level detection result of said level detectionmeans, a level of said modulated input signal subject to threshold valueoperation is a set level; and an adder circuit comprising two rectifyingcircuits for respectively rectifying AC input signals, which adds arectified output from a former stage rectifying circuit to a rectifiedoutput from a latter stage rectifying circuit, and outputs the addedvalues of the two AC input signals from the latter stage rectifyingcircuit, wherein the addition output from said adder circuit is suppliedto an input terminal of said threshold value operation circuit as theinput signal subject to threshold value operation, and an input terminalof the latter stage rectifying circuit of said adder circuit is made ahold input terminal, and an input terminal of the former stagerectifying circuit is made a trigger input terminal, and the AC outputsignal from said threshold value operation circuit is rectified and thenfed back to an output terminal of said former stage rectifying circuit.24. A start signal generating circuit comprising: a start switchcomprising a first contact point and a second contact point having acomplimentary relation to each other for on/off switching; a capacitorwhich is charged via the first contact point which comes on when saidstart switch is pressed, and which is discharged via the second contactpoint which comes on when said start switch returns, for generating anoutput signal; and a threshold value operation circuit which comprises:modulation means for modulating an input signal by means of a switchingsignal from an AC (alternating current) source; and level detectionmeans for level detecting the modulated input signal by comparing themodulated input signal with a previously set threshold value, andgenerating an AC signal when based on a level detection result of saidlevel detection means, a level of said modulated input signal subject tothreshold value operation is a set level; wherein the output signal fromsaid capacitor is made the input signal subject to threshold valueoperation of said threshold value operation circuit, and the output fromthe threshold value operation circuit is made a start signal for amachine.
 25. A start signal generating circuit comprising a first and asecond threshold value operation circuits each of which comprises:modulation means for modulating an input signal by means of a switchingsignal from an AC (alternating current) source; and level detectionmeans for level detecting the modulated input signal by comparing themodulated input signal with a previously set threshold value, andgenerating an AC signal when based on a level detection result of saidlevel detection means, a level of said modulated input signal subject tothreshold value operation is a set level, wherein an AC output from thefirst threshold value operation circuit is input to the modulation meansof the second threshold value operation circuit as the switching signalinstead of the AC source, and wherein a power source voltage level issubject to a threshold value operation by the first threshold valueoperation circuit, and an output signal which is generated based on anON operation of an operation button is made the input signal subject tothreshold value operation of the second threshold value operationcircuit, and the output from said second threshold value operationcircuit is made the start signal for a machine.
 26. A start signalgenerating circuit comprising a self hold circuit provided with twothreshold value operation circuits each of which comprises: modulationmeans for modulating an input signal by means of a switching signal froman AC (alternating current) source; and level detection means for leveldetecting the modulated input signal by comparing the modulated inputsignal with a previously set threshold value, and generating an ACsignal when based on a level detection result of said level detectionmeans, a level of said modulated input signal subject to threshold valueoperation is a set level, wherein one threshold value operation circuitis made a trigger circuit and the other threshold value operationcircuit is made a hold circuit, and an AC signal which is generatedbased on a logical sum operation on an output from said trigger circuitand an output from said hold circuit is input to a modulation means ofsaid hold circuit as the switching signal instead of an AC source, andwherein an output signal generated in accordance with the on/offoperation of a start switch comprising first and second contact pointshaving a complimentary relation to each other for on/off switching, isinput to said trigger circuit of said self hold circuit as the inputsignal subject to threshold value operation, and only when the inputsignal subject to threshold value operation is a set level, and theinput signal level to the hold circuit is a set level, is a start signalgenerated from the self hold circuit.
 27. A threshold value operationcircuit comprising: level detector which level detects an input signalby comparing the input signal with a previously set threshold value;modulator which modulates the level detected signal by a switchingsignal from an AC (alternating current) source; and AC signal generatorwhich generates an AC signal, said AC signal generator being switched byinput of a modulation signal modulated by said modulation means, whereinsaid AC signal is generated when, based on a level detection result ofsaid level detector, a level of said input signal is a set level.